Frequency modulation is a well known modulation technique in communication devices. In most of today's advanced communication devices frequency modulation is accomplished by modulating the frequency of a voltage controlled oscillator (VCO). This is usually accomplished by varying the resonant frequency of oscillation in a tank circuit which is used to generate a carrier signal. Several methods have been utilized for achieving the modulation. The most common way is the use of a modulating varactor placed in series or in parallel with the tank circuit. The varactor will change its capacitance in proportion to an applied modulation signal resulting in a slight variation in the capacitance of the tank circuit. This change in capacitance causes the oscillator frequency to change in response to the modulation signal, resulting in the frequency modulation of the carrier signal. This technique is highly sensitive and in general tends to degrade the Q of the tank circuit. To alleviate the Q degradation problem, the modulating varactor and its associated components must be added to the high Q substrate used to carry the VCO components. However, the increase in the size of the substrate will considerably add to the cost of the VCO.
An alternative to this technique is the use of the control line as the modulation input port. The problem with this modulation method is that there are poles and zeros introduced in the loop filter which results in poor audio response. Another problem with this technique is its temperature dependent characteristics.
With limitations observed on available modulation techniques, it is clear that a need exits for a modulation technique to be effectively used in communication devices without sacrificing cost, space, or sensitivity.